Diffusion bonded structure and method of making

ABSTRACT

Refractory metals or ceramics are diffusion bonded or densified by assembling the workpieces to be bonded, wrapping the assembly with carbon yarn, heating the wrapped assembly, cooling the assembly and unwrapping. The expansion of the workpieces together with the shrinking of the carbon yarn produces tremendous pressures which cause bonding and densifying of the workpiece.

FIELD OF THE INVENTION

This invention pertains to a method of bonding refractory metals andceramic material, more particularly to diffusion bonding under heat andpressure of such materials.

BACKGROUND OF THE INVENTION

It is often necessary to join fine wires of refractory metals inelectron tube structures. Methods using a second metal or material areundesirable for the properites of the alloyed joints they produce.Similarly, it is often desired to join ceramics without the use ofcements or solders to avoid the contamination that the interveningmaterial introduces.

In one prior art method, nickel plated thoriated tungsten wires arewrapped tightly onto an alumina mandrel to form a mesh. The ends of themesh are held onto the mandrel with nickel plated tungsten andmolybdenum tie wires while the mesh is diffused in a hydrogen furnace at1350° C. In the furnace the nickel plating diffused into the tungsten,forming bonds between the crossing wires. This procedure has thedisadvantage that the nickel must be evaporated from the wire beforecarburizing, and while doing so, some Ni atoms diffuse into thetungsten. These foreign atoms lower the recrystallization temperature ofthe tungsten, embrittle the wire, and lower the strength of the wire.

Another prior art method used spot welding of wires at theirintersections. This method, as shown in U.S. Pat. Nos. 3,737,711 and3,724,424 introduces considerable strain because of the deformation fromwelding distorts the mesh.

OBJECT OF THE INVENTION

It is an object of the invention to describe a method of joining aluminaor refractory metals together without any intervening material at theinterface between the parts.

Another object of the invention is to describe a method of joining finewires together while eliminating further steps before carburizing thewires.

SUMMARY OF THE INVENTION

This object of the invention and other objects, features and advantagesto become apparent as the specification progresses are accomplished bythe invention, according to which, briefly stated, the clean ceramicfaces or refractory metal pieces are held together with several layersof carbon yarn. This structure is then heated in a high temperatureoven. The difference of coefficient of expansion of the workpiece andthe carbon yarn creates a pressure which when combined with the heatcauses a diffusion bonding without intervening material. A carburizingof metal also takes place simultaneously.

These and further constructional and operational characteristics of theinvention will be more evident from the detailed description givenhereinafter with reference to the figures of the accompanying drawingswhich illustrate preferred embodiments and alternatives by way ofnon-limiting examples.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of the formation of a bonded grid filamentaccording to the method of the invention.

FIG. 2 is a schematic of the bonded grid formed according to the method.

FIG. 3 is a perspective view of a pair of workpieces to be bondedaccording to the method.

FIG. 4 is a perspective view of the pair of workpieces of FIG. 3 joinedby wrapping with carbon yarn.

FIG. 5 is a perspective view of the finished piece joined according tothe method of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings wherein reference numerals are used todesignate parts throughout the various figures thereof, there is shownin FIGS. 1 and 2 wires 10 of tungsten or other refractory metal arewrapped tightly on a mandrel 12 made of suitable materials such asalumina or silicon carbide to form a mesh filament 14. In contrast tothe method of the prior art described above, the wires 10 are not nickelplated. Bands 16 should be wrapped onto the mandrel 12 before the wire10. The bands 16 facilitate mounting the mesh filament 14 onto the tubeby providing a strip of metal which can easily be spot-welded tosupports. The wires 10 are also diffusion bonded to the bands 16. Thebands 16 can be made of the same refractory metal as the wires 10 or adifferent material such as molybdenum bands with tungsten wire. The endsof the carbons yarn are tied with a square knot and then coated with adab of Loctite TAK-PAK (trademark) adhesive. This adhesive hardens inless than 10 seconds and is very handy in making temporary joints whilewrapping a filament. The adhesive thermally decomposes in the furnace atlow temperatures.

Following the wrapping of the wire, the mesh filament 14 on the aluminamandrel 12 is then covered with two continuous layers of carbon yarn 18(shown in part). The yarn 18 is wrapped tightly so that it squeezes eachcrossing wire in the mesh filament 14 when the mesh filament 14 isdiffusion bonded at a temperature range of 1300° to 1600° C. in ahydrogen, vacuum or inert atmosphere. A bond 20 is formed at theintersection of the wires 10. Experience shows that the yarn exerts aninward radial force on the mesh filament because each segment of the topset of wires which is suspended between the bottom set of wires is bentinward. Since there is no nickel plating on the wire the diffusion is anexchange of tungsten atoms from one wire with those in the adjacentwire. This action is known as self-diffusion.

The method can also be practiced with plated wires, such as nickelplated tungsten, in which case joints can be made at much lowertemperatures still, such as 800°-900° C. This may be useful for otherapplications where it is not so important to eliminate the nickelalloys.

It has also been found that while the bonds are being made, the wire isbeing carburized. Since the carburization is occurring while the meshfilament 14 is stretched on the mandrel and at such a low temperature,1500° C. versus 2200° C. normally, the resulting filament is strongerand rounder than a spot welded filament which is later carburized. Ifthe bonding is accomplished at lower temperatures, it may be necessaryto carburize the mesh filament 14 during a second operation to obtain adesired carbide thickness.

Spot welding of the crossing wires is the most popular method offabricating a mesh filament. This process introduces considerable straininto the mesh because the deformation from welding distorts the mesh.The mesh filament of the prior art is also distorted by carburizingbecause tungsten carbide occupies a volume 11% greater than thetungsten. These two causes of distortion are not present in thedescribed method because bonding of all of the joints and carburizing ofthe wire are occurring simultaneously. The filament wire is strongerbecause its microstructure is fibrous, similar to that of cold drawntungsten wire. The resulting mesh filament is round without any storedstrain energies which will accelerate distorting it when the meshfilament is thermal cycled.

Many mesh filaments have been made in the temperature range of 1390° C.to 1525° C. One of these mesh filaments has been cycled on and off andhas accumulated 3000 cycles. There is no noticeable distortion of thismesh filament as would be obvious if the mesh filament had been spotwelded.

The carbon yarn used in reducing the invention to practice was "BraidedCarbon Cordage" sold by Fiber Materials, Inc. of Biddleford, Me. Thesecords are sold in yarns of 4K (4000), 12K, 18K and 24K individualfilaments in a braided structure. The characteristics of interest inselecting the number of filaments is the uniformity desired in applyingpressure to the assembly. The wraps of a 4K yarn are 3 times denser thanthe 24K yarn and should therefore apply a more uniform pressure to theassembly.

Preloading the yarn makes it possible to practice the method at lowertemperatures. Mesh filaments of tungsten have been made at a temperatureas low as 1390° C. using preloaded yarn. It is believed that pressurealone does not accelerate the diffusion. Rather, the function of thepressure is to bring two surfaces into atomic contact with one anotherso that atoms can be exchanged across the interface. An additionalbenefit of a high pressure is that the wires are deformed in the regionof tangential contact. This deformation increases the area of contactfrom a point to an area large enough that the filament is rugged enoughto be manually handled while assembling the mesh filament into a tube.

It is not clear at this time what the coefficient of expansion of thecarbon yarn used in the invention is, but it is clear that thecoefficient of expansion is either negative, or if positive, very small.From the functioning of the method of the invention it is clear that thecoefficient of expansion of the carbon yarn is much less than that ofthe alumina mandrel. In an aritcle published in Volume 1, Issue 2 of"Advances in Material and Processes", page 25, the coefficient ofexpansion of carbon fiber is given as -0.55×10⁻⁶ /°F.

Bonding could occur at temperatures less than 1000° C. if the materialsbeing joined were more thermally active than tungsten. As a generalrule, most diffusion bonds can be made near the recrystallizationtemperature of the material. The recrystallization temperature isdependent upon such factors as amount of cold work in the metal,impurities, time at temperature, and the starting grain size of thematerial. The recrystallization temperature is important because at thistemperature, the first microstructural changes occur indicating thatthere is sufficient thermal energy available for the atoms to rearrangethemselves. From the knowledge of recrystallization of other materials,it is possible to approximate the temperature at which they may bebonded. Since most materials recrystallize at temperature less thantungsten, most materials could be bonded at lower temperatures thanmentioned in this disclosure. If the tungsten wire is coated with amaterial having a lower recrystallization temperature, it would beexpected that the nature of the coating would dictate the joiningtemperature. If a coating is employed, consideration must then be givento the consequences of its behavior to the system when the system issubjected to temperatures higher than that during bonding.

Unlike materials can be bonded by the method of the invention. Forexample, the structure shown in FIG. 1 can be bonded with tungsten wiresand molybdenum bands. The recrystallization temperature of molybdenum islower than that of tungsten and these metals are soluble in each other.On the other hand, the tungsten wires so not stick to the aluminamandrel bacause the alumina and the metals are not soluble in oneanother. Also a pure grade 99.5% Al₂ O₃ alumina is used so that there ishardly any glassy phase present in the alumina to adhere to thetungsten. In metallizing, it is the glass phase either added to themetallizing paint or to the ceramic which promotes adherence.

The formation of filament structures according to the method of theinvention has several additional advantages beyond the quality of thebonds formed. Rounder filaments can be formed in this way. The deviationfrom round is typically 0.001 inch which is nearly the same as that ofthe mandrel. The mandrel dimensions determine the inside diameter of thefilament. Also, the method of the invention removes the residualstresses in the wires which were caused by drawing and wrapping the wireon the spool on which it was shipped.

The difference in thermal expansion of the alumina mandrel and the yarncauses an additional force to be exerted on the crossing wires when theassembly is heated. Since the yarn is flexible, similar to a piece ofstring, a uniform radial force is applied to the cylindrical surface ofthe mandrel. In this way, if end affects are ignored, it is similar tothe HIP process, hot isostatic pressing. Hot isostatic pressing of partsis the current commercial process for densifying ceramic parts andcastings which have voids in their microstructures. It is not difficultto envision using carbon yarn as a substitute for HIP furnaces. HIP isan expensive process and is only done to critical parts. Parts whichhave been subjected to HIP are stronger and tougher than before thetreatment.

As an example of the method of joining ceramics consider joining twopieces of alumina. It is important not to have a second material in thejoint because it is desirable to retain all of the high temperatureproperties of the alumina in the joint. The joint might be expected tohave a high operating temperature such as 1600° C. and any materialsadded to the joint, such as silica, will lower the softening temperatureof the alumina.

The joint is made by polishing the facing ceramic surfaces in the samemanner as metallographic samples are prepared for examination. Thepolishing entails the use of finer diamond laps until the 1 micron sizelap is reached. Crystal damage from the preceding lap is always removedbefore proceeding to a finer lap. The polished surfaces are then cleanedto remove all residue from polishing. The bond between the two pieces ofceramic is made by wrapping them together with carbon yarn so that thejoining surfaces are held in compression. When the assembly is heatedthe carbon yarn contracts and the parts expand, exerting a tremendousforce on the joining surface. If the temperature is high enough foratomic diffusion, a bond will be made by exchanging atoms on each sideof the interface. This phenomenom occurs rapidly in the temperaturerange of recrystallization or grain growth for the material.

This invention was demonstrated by joining two pieces of Coors AD94 andAD995 ceramics in the form of ASTM; CLM-15 tensile test pieces 30, shownin FIG. 3. The surfaces 32 to be joined were polished and thenultrasonically cleaned. To facilitate the wrapping of the carbon yarn 34as shown in FIG. 4 around the abutting pieces, four notches 36 were cutinto the flange ends of the parts at every 90°. The notches preventedthe yarn from sliding off the edge of the pieces. Between 30 to 50 wrapsof carbon yarn were wrapped around the abutting parts, alternatingbetween two sets of notches 36. Additional yarn was wrapped around themid-section of the assembly 90° to the first wraps, to pull the bondingyarn tighter and closer to the OD Of the assembly. This assembly wasthen fired in a wet 75/25 nitrogen hydrogen atmosphere at 1525° C. for1/2 hour.

After firing the parts, the carbon yarn was cut loose from the assemblyas shown in FIG. 5. The yarn was stretched and frail from reacting withthe furnace atmosphere. The two pieces of ceramic were joined into asingle piece 38 and were vacuum tight.

This invention is not limited to the preferred embodiments andalternatives heretofore described, to which variations and improvementsmay be made including mechanically and electrically equivalentmodifications, changes and adaptations to component parts, withoutdeparting from the scope of production of the present patent and truespirit of the invention, the characteristics of which are summarized inthe appended claims.

What is claimed is:
 1. A method of diffusion bonding of a firstworkpiece to a second workpiece, comprising the steps of:contacting afirst workpiece to a second workpiece to form a composite structure;wrapping said composite structure in carbon yarn to form a wrappedstructure; and heating said wrapped structure to a temperaturesufficient to cause diffusion bonding between said first workpiece andsaid second workpiece.
 2. The method of claim 1 wheren said temperatureis at least the recrystallization temperature of said first and saidsecond workpieces.
 3. The method of claim 1 wherein said first and saidsecond workpieces are wrapped over a mandrel, said mandrel being made ofmaterial insoluble in the materials of said first and said secondworkpieces.
 4. The method of claim 1 including the subsequent stepsof:cooling said wrapped structure; and unwrapping said carbon yarn fromsaid composite structure.